14 FROM FISH TO PHILOSOPHER 



creases the rate of mutation, but whether the mutations 

 SO produced are those that might arise spontaneously is 

 not determined. It has been inferred that cosmic radia- 

 tion may in part be responsible for 'spontaneous' muta- 

 tion, but the idea is as yet without support in evidence. 

 A few chemicals such as colchicine and the nitrogen 

 mustards also produce mutations, probably by reacting 

 chemically with the chromosomes, but again there is no 

 evidence that such compounds are responsible for the 

 naturally occurring process. Consequently when we call 

 mutations 'spontaneous' we are only admitting that the 

 underlying causes are unknown. 



Mutations are also said to be 'random' in nature: 

 though limited in quality and number, they may take 

 any one of several forms, and in the great majority of 

 instances they have no adaptive value to the organism 

 and may even be lethal. If a mutation is useful, it is so 

 by chance alone. Hence, while random mutation supphes 

 the raw materials of evolution, mutation alone would 

 be of httle avail in modifying organic pattern in the di- 

 rection of better adapted or more complex animals 

 were it not for the supplementaiy operation of natural 

 selection. 



For Darwin, natural selection was a rather crude proc- 

 ess of competition between the members of a species, 

 which worked to eliminate the 'unfit' and to permit the 

 survival of the 'fit' in the perpetual struggle for existence. 

 But modem biology sees in natural selection a far more 

 complex process, and one that begins to operate between 

 the genes and chromosomes from the instant when the 

 sexual cells unite in fertiHzation. It has perhaps com- 

 pleted a large part of its work in the embryonic period, 

 determining the issue of death or survival of the develop- 

 ing embryo early in the period of gestation. And in so 

 far as it carries on into adult life, natural selection is 

 much more complex than Darwin supposed: it goes be- 

 yond competition in the grosser sense and includes com- 

 petence to meet environmental extremes of heat and 



